As a technique for the deposition of copper film onto a commercially important substrate, like a silicon wafer, supercritical fluid deposition [“SCFD”], is becoming increasingly important because of its environmental and economic benefits. SCFD is a deposition technique akin to the firmly established technique of chemical vapor deposition [“CVD”]. (See generally, Hitchman et al., eds. (1993) CHEMICAL VAPOR DEPOSITION PRINCIPLES AND APPLICATIONS). SCFD results in copper deposits by heating a solution of an organometallic precursor dissolved in a supercritical fluid.
Supercritical fluid deposition techniques have been reported for about fifteen years. For a synopsis of the history of these techniques, see Ye, X and Wai, C M (2003) Making Nanomaterials in Supercritical Fluids: A Review, J Chem Ed 80(2): 198, 201-203. Generally, early supercritical fluid deposition reports demonstrated a variety of chemical deposition mechanisms, such as thermolysis (see Louchev, O A, Popov, V K, Antonov, E and Lemenovski, D A (1995) Crystal Growth 155: 276 and Bocquet, J F, Chhor, K and Pommier, C (1994) Surf and Coat Tech: 70, 73); or hydrolysis (Brasseur-Tilmant, J, Jestin, P and Pommier, C (1999) C MaterRes Bull 34: 2013); or oxidation/nitridation (see EP Patent EP1024524 (2000) to Morita, K, Ohtsuka, T and Ueda, M); or precipitation (see U.S. Pat. No. 4,737,384 (1988) to Murthy et al.) or expansion of a precursor-supercritical solvent solution to fine aerosol which was then injected into a typical CVD reactor (see U.S. Pat. No. 4,970,093 (1990) to Siever et al.)
Recently, Watkins et al. developed a kind of supercritical fluid deposition called chemical fluid deposition [“CFD”], which is a hybrid method that blends the advantages of chemical vapor deposition and electroless plating. See U.S. Pat. No. 6,689,700 (2004) and U.S. Pat. No. 5,789,027 (1998), both of which are hereby incorporated herein by reference. Generally, in CFD the organometallic precursor is dissolved in a supercritical or near-supercritical solution, which is heated to bring about a deposition reaction onto a substrate in contact with the solution. Watkins et al. discusses that metallic precursors may contain variously palladium, platinum, copper, or nickel; that chemical reactions may include reduction, oxidation, or disproportionation; and that substrates may include silicon wafers or a porous solid substrate.
Specifically, deposition of copper via CFD may occur in a two-reaction process in which a seed layer of copper is first deposited onto the substrate and then additional copper is added to the seed layer. The first of the two deposition reactions is typically a disproportionation reaction having the form:
in which L is a neutral ligand and comprises a Lewis base and n may be 1, 2 or a decimal. In this reaction, the copper-containing Cu(I) precursor may be dissolved in a solvent, already at or near a supercritical state. Alternatively, the precursor-solvent may be brought to supercritical conditions after dissolution. In either case, the substrate is heated, which brings about a thermal reduction, i.e., a disproportionation reaction, and results in nonselective deposition of a seed layer of copper onto the substrate surfaces in contact with the solution.
The second reaction is typically a chemical reduction of the disproportionation by-product, Cu(II) (β-diketonate)2 and is brought on by the addition of a reduction reagent, such as H2. In general, the chemical reduction reaction also takes place at the temperature of the disproportionation reaction. The chemical reduction results in selective deposition only onto the copper seed layer surface. This two-step process may also occur simultaneously by adding reducing agent to the supercritical solution before heating.
CFD has great potential in fabricating, especially, copper films or copper wires of widths below 100 nanometers, which serve as interconnects between transistors in integrated circuit boards. Even though olefin and acetylene complexes of β-diketonate copper precursors are generally known, there is still a need to run the deposition reaction at a lower temperature than previously disclosed, for example, lower than disclosed in Watkins et al., supra, for its Cu(hfac) (2-butyne) complexes. This is because the Cu(II) by-products are less likely to decompose at the lower reaction temperature. It is also believed that the Cu(II) by-products are more soluble in scCO2, the preferred supercritical solvent, than their decomposition products. Thus, to maximize utility of CFD for copper deposition, there is a need to use copper-containing precursors, which are not halogenated, and which result in Cu(II) by-products that remain in the supercritical solution.